This invention relates to the compression of pictorial data such as video and still image data, and more specifically to the compression coding of the higher frequency subbands in a system which decomposes, in two dimensions, a still image or video signal.
The principle of subband coding is based on the decomposition of an input still image/video frame, into a subpicture/subframe, where each represents a region in the two-dimensional frequency spectrum of the image/video frame. The main property of subband decomposition is the ability to reshape the noise in the frequency spectrum by taking into consideration human visual perception. The lowest frequency band contains the general brightness areas of the picture whereas the edge information falls within the higher frequency bands. It is, however, a characteristic of the eye (spatial masking) that it is unable to perceive errors in the vicinity of large changes in the brightness. To exploit this important visual property and achieve greater compression, it is desirable to encode the high frequency bands with lower accuracy. In other words, coding efficiency depends heavily on how these higher frequency bands are coded. As far as the lowest band is concerned, since this band (in terms of the number of samples relative to the original) is a "miniature" version of the original image/video frame, there is little that can be done except to employ some conventional coding methods such as differential pulse code modulation (DPCM), transform coding, or hybrid coding, etc.
Two-dimensional quadrature mirror filtering (QMF) of image and video signals has been studied extensively in the prior art. The application of QMF to image coding has been considered by J. W. Woods and S. O'Neal in "Sub-band Coding of Images," Proc. ICASP 86, April 1986, pp. 1005-1008 and in a similarly titled article by the same authors in IEEE Trans. ASSP, Vol. 34, October 1986, pp. 1278-1288. It has also been investigated by the inventor herein and A. Tabatabai in several articles such as "Sub-band Coding of Digital Images Using Two-Dimensional Quadrature Mirror Filtering," Proc. SPIE, Vol 707, September 1986, pp. 51-61; in "Applications of Quadrature Mirror Filtering to the Coding of Monochrome and Color Images," proc. ICASP'87, Vol. 4, pp. 2384-2387; and in "Subband Coding of Monochrome and Color Images," IEEE Trans. on Circuits and Systems, Vol. 35, February 1988, pp. 207-214. In the Woods and O'Neal approach, individual bands are coded using adaptive DPCM, whereas in the approach described by the inventor herein and A. Tabatabai, except for the lowest frequency band, all other bands are coded using a combination of PCM quantization and run-length coding for the transmission of the location of nonzero PCM values. Subband coding of video signals has also been considered in the inventor herein in U.S. Pat. No. 4,969,010 issued Nov. 6, 1990. As described in that patent, differential pel values are decomposed using separable two-dimensional quadrature mirror filtering. Each subband is quantized separately and coded by entropy coders, which code the quantized values by variable word-length coding the nonzero quantized values and transmitting that information with the corresponding run-length coded positional information.
In the prior art schemes described in the aforenoted articles relating to image compression by the inventor herein and A. Tabatabai, and in the aforenoted patent relating to compression of a video signal, each of the high frequency decomposed bands are quantized using individual uniform quantizers having a center dead-zone and a step-size that may be tailored to the characteristics of the particular band. The function of the quantizer dead-zone is to eliminate the low level noise which would consequently reduce the number of nonzero values. As a result, the higher band images and video signals contain only a limited number of nonzero pels, which correspond to the contour points in the still image or video frame. In order to achieve better compression, only the nonzero quantized values with their corresponding positional information need be transmitted. In particular, and as noted above, the positional information is run-length coded by considering each scan line as a sequence of black and white runs, wherein the black run corresponds to zeros and the white run to nonzero values. In these prior art schemes, a significant number of overall bits go toward the run-length coded positional information.
An object of the present invention is to improve the coding performance by increasing the efficiency in which the positional information is coded for the higher frequency bands of a two-dimensional QMF decomposed pictorial data derived from a still image or video signal.
An additional object of the present invention is to exploit the two-dimensional spatial dependencies in an image or video frame in the coding of the positional information.
A further object of the present invention is to devise a two-dimensional run-length coding strategy which provides a signal with long bit runs.